I have already shared some of the fun and games maintaining my 26" "townie" comfort bike.
-replaced rear derailleur and shift cable
-replaced chain, required combining two stock chains because townie has rear wheel set back several inches.
-replaced tires when sidewall failed. Original 1.9" tires failed, I bought one pair of 1.5" 65# tires (on bike now), and a pair of 1.85" 87# tires.
I like the comfortable (wider) saddle, and lower seat height but the easy gearing was not up to the fast downhill on my normal circuit.
The stock 7 speed rear freewheel (sprocket) had a 14t high gear, and 34t low gear. The 34t low, was so low I never used the smallest (granny) 30t front ring since I bought the bike (used) to climb the steepest hills on my course. I replaced the stock "comfort" freewheel with a 11t-28t, significantly faster at both ends. Now the granny ring will get used and deliver roughly the same lowest hill climb ratio as before on the middle ring.
I thought I was replacing the front 3-ring pedal crank assembly with the same ratios but counting teeth, the old big ring was 48t and the replacement is 50t so that is 4% faster too. The combined high gear upgrade is now more than 30% faster. Friday was my first full road test and it is as good as I expected. I had a strong headwind on the fast downhill so didn't get to see how fast it could really go, but fast enough to make me happy.
I ended up buying some specialized bike tools, but tools and parts are still far cheaper and easier than carrying it to the nearest bike shop 25 miles away, and paying full retail for parts plus labor. Next time I will already have the tools.
For future projects, in the near term I still have a new set of 1.85" 87# rated tires to check out. Longer term I am thinking about buying a new rear wheel. The current rear wheel has a few spokes that need adjustment (truing). The cheaper freewheel hub offers less gearing options, and is less robust (axles reportedly get bent easily). So I am tempted to eventually replace the rear wheel with one that uses the better splined freehub, that accepts a wider range of modern cassette rear sprockets. I had to buy a no-name chinese freewheel because shimano (the big dog in bike hardware) stopped making a freewheel with faster ratios.